Polysulphone film thickness and the effects on the dose calibration in ultraviolet
radiation dosimetry were investigated. Compared to those obtained with the dose
calibration for the 45 mm thick film the erythemal exposures
determined from the dose calibrations for the 18, 20 and 30 m m
thick film ranged from 33% to -45%. The absolute of the differences averaged to 22%, 37%
and 19% for the 18, 20 and 30 mm thick respectively. The
variations in the dose response of polysulphone film with different thickness have shown
that the dose response is highly dependent on the film thickness and highlighted the
importance of employing polysulphone film of consistent and reproducible thickness in
ultraviolet photobiology research.

1. Introduction

Photobiological research to determine the ultraviolet (UV) exposure to humans
requires the personal monitoring of UV exposure. The usage of biological and chemical
dosimeters has been previously reviewed (Wong and Parisi, 1998, Horneck, 1995). A common
technique is to employ a photosensitive film and the UV induced deterioration of the film
is calibrated to the measured UV irradiance. A commonly employed film dosimeter material
in ultraviolet photobiology research is the polymer polysulphone (Davis et al., 1976,
Diffey, 1984).

The film employed in the research in the literature is generally of the order of 40 mm thick. This thickness is generally employed as a compromise
between ease of handling and matching of the spectral response of the film (CIE, 1992) to
the human erythemal action spectrum (CIE, 1987). A polysulphone film of 1 mm thickness on a cellophane substrate has been reported to have a
different spectral response to 40 mm thick film (Davis et al.,
1981). Consequently, polysulphone film of different thickness may have a different dose
response. Variations in film thickness may occur in the film casting process and introduce
significant errors in the dose response of the film. No previous research has
quantitatively assessed the effect of the polysulphone film thickness in UV radiation
dosimetry. This paper investigates the effects on UV exposures measured with polysulphone
film of different thickness.

2. Materials and Methods

2.1 Film Thickness

The polysulphone film of different thicknesses was produced at the University of
Southern Queensland (USQ) Centre for Astronomy and Atmospheric Research. The technique
developed with a specifically designed casting table ensures production of consistent and
reproducible sheets of polysulphone film. The technique is similar with some variations to
that utilised by the original producer (Davis et al., 1976). Polysulphone pellets (Aldrich
Chemical Co., Inc. Milwaukee WI 53233 USA) were mixed with chloroform to form a solution
and cast on a glass slab optically flat to 1 mm by a moving
blade powered by a variable speed DC motor. The height of the blade is adjustable to
produce the required thickness.

The average thickness of the film was calculated employing the technique of Davis et
al., (1981). This involves weighing a known surface area of the film and employing the
density of polysulphone of 1.2 g cm-3 (Davis et al., 1981). Four film
thicknesses of 18, 20, 30 and 45 mm were produced by setting
the height of the casting blade above the glass slab to different values. The film was
stored in the dark prior to use to prevent UV exposure prior to usage.

2.2 Calibration

The erythemal UV exposure, UVery can be expressed by weighting the source
spectrum S(l) with the CIE (1987) action spectrum for human
erythema, A(l), as follows:

(1)

The UVery may be measured with a calibrated spectroradiometer measuring S(l) or with a detector that possesses a sensitivity approximating the
erythemal action spectrum. In this research, the polysulphone was calibrated against a
temperature stabilised calibrated Biometer (model 501, Solar Light Co., Inc. 721 Oak Lane,
Philadelphia, PA. 19126) in units of MED or minimum erythemal dose. One MED is the amount
of biologically effective UV required to produce barely perceptible erythema after an
interval of 8 to 24 hours following UV exposure (Diffey, 1992). The polysulphone was
calibrated to the spring solar spectrum in October between approximately 11:00 Australian
Eastern Standard Time (EST) and 13:00 EST. The Biometer was calibrated to a
spectroradiometer with calibration traceable to the National UV Standard housed at the
CSIRO National Measurement Laboratory. From the calibration the unit of one MED on the
Biometer was equal to 210 J m-2.

The film was fabricated into dosimeters of overall approximate size 3 cm x 3 cm with an
aperture of approximately 1 cm2. A total of 48 dosimeters with 12 each
constructed from film of the four thicknesses were employed. The complete batch of
dosimeters for the four thicknesses was calibrated at once so that all four thicknesses
were exposed to the same source spectrum. A photograph of the UV Biometer and a
polysulphone dosimeter are provided in Figure 1. The calibration was performed on a
horizontal plane in an unshaded position at Toowoomba (27.5 oS), Australia.
Following the established procedure (Diffey, 1989), the optical absorbance of the film was
measured pre- and post-exposure at 330 nm in a spectrophotometer (Shimadzu Co., Kyoto,
Japan) to determine the change in optical absorbance at 330 nm (DA330)
due to the UV exposure. The absorbance was measured at four sites over the film in order
to take into account any minor variations in the film over the dosimeter. The dark
reaction was taken into account by allowing a constant time between exposure and read-out
and measuring the post-exposure absorbance on the following day.

To see the full size image (47k) click on the picture above

Figure 1  Photograph of the UV Biometer and a polysulphone dosimeter.

3. Results

3.1 Comparison

In Figure 2, the calibration data of the current film of 45 mm
thickness is compared to the calibration in the literature. For a film thickness of 40 ± 4 mm, Diffey (1989) obtained a dose
calibration of:

UVery =
2000[9(DA330)3 + (DA330)2
+ (DA330)]
(2)

in units of J m-2. In Figure 2, the factor of 2000 in Equation (2) has been
converted to produce units of MED for the solid line in the Figure. The average difference
between the calibration data points and the calibration in the literature (Diffey, 1989)
compared to the literature calibration is 7%.

The optical absorbances at 330 nm before exposure averaged over the four measurement
sites and the twelve dosimeters were 0.530 ± 0.032, 0.530 ± 0.014, 0.543 ± 0.010 and 0.587 ± 0.011 for the film thicknesses of 18, 20, 30 and 45 mm respectively. The error is represented as one standard deviation.
There is a statistically significant difference (Students t-test) in the optical
absorbance at 330 nm between the 45 and the 30, 20 and 18 mm
thicknesses (P < 0.05). Similarly, there is a statistically significant difference
between the 30 and the 20 and 18 mm thicknesses with no
statistical difference between the 20 and 18 mm thicknesses.

Figure 2  Comparison of the calibration data for the 45 m
m thick film (· ) to that obtained by previous research (solid
line) (Diffey, 1989).

3.2 Calibration

The dose calibrations for spring sunshine of the different polysulphone film
thicknesses are provided in Table 1. Each of the calibrations have R-squared values
between 0.92 and 0.98. The dose calibrations for the 20 mm and
the 45 mm thick film are plotted in Figure 3 and show the
differences between the calibrations for the two film thicknesses. The dose calibrations
are provided to allow comparison of the dose response for the different thicknesses. They
are only applicable for the solar UV spectrum encountered in this research as previous
research has established that for a different source spectrum, the polysulphone has to be
re-calibrated for that spectrum in order to minimise the errors (Wong et al., 1995).

Table 1  Dose calibrations for spring sunshine of the different polysulphone film
thicknesses

Thickness (mm)

Erythemal UV Exposure (MED)

18

-90.08(DA330)3
+ 120.82(DA330)2  3.36(DA330)

20

-7.88(DA330)3
+ 58.49(DA330)2 + 9.75(DA330)

30

-102.11(DA330)3
+ 121.50(DA330)2  3.26(DA330)

45

87.58(DA330)3
+ 2.94(DA330)2 + 10.47(DA330)

Figure 3  Calibration data for the 45 mm thick film (x)
and the regression curve (1) and the calibration data for the 20 mm
thick film (n) and the regression curve (2).

The dose calibration equations in Table 1 have been employed for each of the four
thicknesses to provide the erythemal exposures for seven different scenarios of DA330 from 0.1 to 0.4 in steps of 0.05 as listed in column
1 of Table 2. The erythemal exposures are in the next four columns of Table 2. Different
erythemal exposures are produced for the different film thicknesses. For example, for a DA330 of 0.2, the erythemal exposure measured is 4.2 MED
for the 20 mm film compared to 2.9 MED for the 45 mm film. The final three columns in Table 2 show the percentage
differences in the erythemal exposures with the 18, 20, and 30 mm
film dose calibrations compared to the 45 mm film dose
calibration. The percentage errors range from 33% to 45% compared to the erythemal
exposures for the 45 mm film dose calibration.

Table 2  The erythemal exposures for the film thicknesses for different scenarios of
change in absorbance and the percentage differences compared to the exposures measured
with the 45 mm thick film.

D A330

Erythemal Exposures (MED)

% differences compared to 45
mm

18 mm

20 mm

30 mm

45 mm

18 mm

20 mm

30 mm

0.10

0.78

1.6

0.79

1.2

33

-33

32

0.15

1.9

2.8

1.9

1.9

1

-42

2

0.20

3.4

4.2

3.4

2.9

-18

-45

-16

0.25

5.3

6.0

5.2

4.2

-27

-43

-24

0.30

7.4

8.0

7.2

5.8

-29

-38

-25

0.35

9.8

10.2

9.4

7.8

-25

-32

-20

0.40

12.2

12.8

11.6

10.3

-19

-24

-13

4. Discussion

The comparison of the calibration of the 45 mm thick
polysulphone film with the polysulphone film calibration in the literature verifies the
quality of the current batch of film as it possesses a similar dose calibration to that
reported by Diffey (1989). However for different film thicknesses, the dose calibration
changes. The percentage differences in the erythemal exposures determined from the dose
calibrations for the 18, 20 and 30 mm thick film, compared to
those obtained with the dose calibration for the 45 mm thick
film ranged from 33% to -45%. The absolute of the differences averaged to 22%, 37% and 19%
for the 18, 20 and 30 mm thick film respectively. The
variations in the erythemal exposures obtained with polysulphone dosimeters of different
thickness have shown that the dose response is highly dependent on the film thickness and
highlighted the importance of employing polysulphone film of consistent and reproducible
thickness in UV photobiology research. The polysulphone film casting table employed at the
USQ with reproducibly settable casting blade heights and casting blade speeds ensures
this.

Acknowledgements - The authors would like to thank
Ken Mottram, Oliver Kinder and Graeme Holmes in the USQ physics discipline whose technical
expertise contributed to this project.